In wireless communication systems, downlink reference signals are normally created to provide reference for channel estimation used in coherent demodulation as well as a reference for a channel quality measurement used in multi-user scheduling. In the LTE Rel-8 specification, one single type of downlink reference format called a cell-specific reference signal (CRS) is defined for both channel estimation and channel quality measurement. The characteristics of Rel-8 CRS include that, regardless of multiple in, multiple out (MIMO) channel rank that the user equipment (UE) actually needs, the base station can always broadcast the CRS to all UE based on the largest number of MIMO layers/ports.
In the 3GPP LTE Rel-8 system, the transmission time is partitioned into units of a frame that is 10 ms long and is further equally divided into 10 subframes, which are labeled as subframe #0 to subframe #9. While the LTE frequency division duplexing (FDD) system has 10 contiguous downlink subframes and 10 contiguous uplink subframes in each frame, the LTE time-division duplexing (TDD) system has multiple downlink-uplink allocations, whose downlink and uplink subframe assignments are given in Table 1, where the letters D, U and S represent the corresponding subframes and refer respectively to the downlink subframe, uplink subframe and special subframe that contains the downlink transmission in the first part of a subframe and the uplink transmission in the last part of subframe.
TABLE 1TDD allocation configurationsDownlink-to-UplinkUplink-Switch-downlinkpointSubframe numberconfigurationperiodicity012345678905 msDSUUUDSUUU15 msDSUUDDSUUD25 msDSUDDDSUDD310 ms DSUUUDDDDD410 ms DSUUDDDDDD510 ms DSUDDDDDDD65 msDSUUUDSUUD
In one system configuration instance (called normal cyclic prefix, or normal-CP) in LTE, each subframe includes 14 equal-duration time symbols with the index from 0 to 13. The frequency domain resource, up to the full bandwidth within one time symbol, is partitioned into subcarriers. One physical resource block (PRB) is defined over a rectangular 2-D frequency-time resource area, covering 12 contiguous subcarriers over the frequency domain and 1 subframe over the time domain, and holding 12*14=168 resource elements (RE), as shown in FIG. 2, for example. In addition, each subframe can also contain two equal-length slots, with each slot containing 7 OFDM symbols. In normal-CP configuration, the OFDM symbols are indexed per slot, where the symbol index runs from 0 to 6; the OFDM symbols can be also indexed per subframe, where the symbol index runs from 0 to 13.
Each regular subframe is partitioned into two parts: the PDCCH (physical downlink control channel) region and the PDSCH (physical downlink shared channel) region. The PDCCH region normally occupies the first several symbols per subframe and carries the handset specific control channels, and the PDSCH region occupies the rest of the subframe and carries the general-purpose traffic. The LTE system requires the following downlink transmissions to be mandatory:
Primary synchronization signal (PSS) and secondary synchronization signal (SSS): These two signals repeat in every frame and serve for the initial synchronization and cell identification detection after UE powers up. The transmission of PSS occurs at symbol #6 in subframes {0,5} for FDD systems with normal-CP, and at symbol #2 in subframes {1,6} for TDD systems; the transmission of SSS occurs at symbol #5 in subframes {0,5} for FDD with normal-CP, and at symbol #13 in subframes {0,5} for TDD with normal-CP;
Physical broadcast channel (PBCH): PBCH also repeats in every frame, and serves for broadcasting of essential cell information. Its transmission occurs over 4 symbols {7˜10} in subframe #0;
Cell-specific reference signal (CRS): CRS serves for downlink signal strength measurement, and for coherent demodulation of PDSCH in the same resource block. Sometimes it is also used for verification of cell identification done on PSS and SSS. CRS transmission has the same pattern in each regular subframe, and occurs on symbols {0,1,4,7,8,11} with a maximum of four transmission antenna ports in a normal-CP subframe. Each CRS symbol carries two CRS subcarriers per port per resource block dimension in frequency domain, as shown in FIG. 2;
System information block (SIB): SIB is the broadcast information that is not transmitted over PBCH. It is carried in a specific PDSCH that is decoded by every handset. There are multiple types of SIB in LTE, most of which have a configurably longer transmission cycle, except SIB type-1 (SIB1). SIB1 is fix-scheduled at subframe #5 in every even frame. SIB is transmitted in PDSCH identified by a system information radio network temporary identifier (SI-RNTI) given in the corresponding PDCCH; and
Paging channel (PCH): The paging channel is used to address the handset in idle mode or to inform the handset of a system-wide event, such as the modification of content in SIB. In LTE Rel-8, PCH can be sent in any subframe from a configuration-selective set from {9}, {4,9} and {0,4,5,9} for FDD and {0}, {0,5}, {0,1,5,6} for TDD. PCH is transmitted in PDSCH identified by the paging RNTI (P-RNTI) given in the corresponding PDCCH.
Note that PSS/SSS/PBCH are transmitted within the six central PRBs in frequency domain, while SIB and PCH could be transmitted at any portion within the whole frequency bandwidth, which is at least six PRBs.
Besides the regular subframe as shown in FIG. 2, LTE systems also define one special subframe type—Multi-Media Broadcast over a Single Frequency Network (MBSFN) subframe. This type of subframe is defined to exclude regular data traffic and CRS from the PDSCH region. In other words, this type of subframe can be used by a base station, for example, to identify a zero-transmission region so that the handset would not try to search for the CRS within this region. The downlink subframes {1,2,3,6,7,8} in FDD and the downlink subframes {3,4,7,8,9} in TDD can be configured as an MBSFN subframe. In this disclosure, subframes are termed MBSFN-capable subframes, while the rest of downlink subframes may be referred to as non-MBSFN-capable subframes. Note that most of the essential downlink signals and channels discussed above (e.g., PSS/SSS, PBCH, SIB and PCH) are transmitted in non-MBSFN-capable subframes.
As 3GPP LTE evolves from Rel-8 to Rel-10 (also called LTE-advance or LTE-A), due to the large number of supported antenna ports (up to 8), it can cost a large amount of overhead to maintain the CRS-like reference signal on all ports. It is agreed to separate downlink reference signal roles to the following different RS signaling:
Demodulation reference signal (DMRS): this type of RS is used for coherent channel estimation and should have sufficient density and should be sent on a per UE basis; and
Channel state information reference signal (CSI-RS): this type of RS is used for channel quality measurement by all UEs and could be implemented over the frequency-time domain.
It is agreed in the 3GPP standard body that: DMRS patterns in each PRB is determined to be located at 24 REs as shown in FIG. 2; CSI-RS RE can not be allocated to symbols carrying PDCCH and Rel-8 CRS (i.e., CSI-RS cannot be allocated to REs on the symbols labeled as “CRS RE on antenna port k” and “Data RE on CRS symbol” in FIG. 2); the CSI-RS can only be inserted in resource elements which will not be interpreted by Rel-8 UEs as PSS/SSS or PBCH; the same CSI-RS pattern is desired between a non-MBSFN subframe and an MBSFN subframe. In other words, the CSI-RS pattern is designed based on the available resources in a non-MBSFN subframe; CSI-RS transmission cycles per cell is an integer multiple of 5ms, and per-cycle transmission of CSI-RS RE for all ports per cell is performed within a single subframe; and NANT is denoted as the number of CSI-RS antenna ports per cell. The average density of CSI-RS is one RE per antenna port per PRB for NANT ∈ {2,4,8}.
Based on these agreements, this disclosure provides further principles and methods to allocate CSI-RS signals, among other features that will become apparent in light of the following description. These and other implementations and examples of the cell identification methods in software and hardware are described in greater detail in the attached drawings and detailed description.